In recent years, there have been proposed motor controllers which have, at the input side of the inverter circuit, a small-capacity capacitor in place of a large-capacity smoothing capacitor in view of resource saving and cost reduction.
FIG. 13 is a circuit diagram showing a configuration of such motor controllers. In the motor controller shown in FIG. 13 (hereinafter referred to as the first prior art), since a capacitor 203 is small in capacity, an input voltage to be applied to an inverter circuit 204, which has been obtained by rectifying the output voltage of an a.c. electric power source 201 with a rectifier circuit 202, cannot been satisfactorily smoothed, so that the input voltage has a pulsating waveform. The voltage with such pulsation synchronizes with the output voltage of the a.c. power source 201 and has frequency twice that of the output voltage of the a.c. power source 201. To cope with this, a desired torque command to be input to a brushless motor 205 is made to have a waveform which is synchronous with and analogous to the input voltage for the inverter circuit 204 as shown in FIG. 14(a). Thereby, the brushless motor 205 can be driven even with a pulsating voltage and the input current I from the a.c. electric power source 201 has a sinusoidal waveform as shown in FIG. 14(b), so that the power factor can be prevented from decreasing (e.g., Japanese Patent Publication Kokai No. 2002-51589 (FIGS. 1 and 9)).
In the case of brushless motors which drive a compressor used for air conditioners, refrigerators, etc., noise and vibration occur especially in a low rotational speed region, owing to great load fluctuation per rotation. Noise and vibration are caused, especially, in rotary-type compressors and reciprocation-type compressors, for the reason that the load torque imposed on the brushless motor largely fluctuates, as shown in FIG. 15, according to the rotational phase (rotor angle) of the motor to be maximum with the timing at which the refrigerant is discharged so that the load torque pulsates while the rotor making one rotation. The pulsation becomes more intense with the average rotational speed decreasing, and the intense pulsation is accompanied with increases in the amplitude of vibration. As an attempt to solve this, there has been proposed a method for controlling motor current so as to reduce vibration taking the load fluctuation into account. In this motor current control method (hereinafter referred to as “the second prior art”), acceleration or a change in speed per rotation is computed from an estimated rotational speed of the motor, and a motor current command (amplitude command) is prepared so as to make the change small. More specifically, the rotational phase of the motor is divided into desired sections and, for every divided section, a torque command correction amount for reducing vibration is prepared from the acceleration or the change in speed to add to the motor current command. In this motor current control method, since the motor current command largely increases or decreases once per rotation of the rotor, the power supply rate of the a.c. power source also largely increases or decreases each time the motor rotates, resulting in a drop in the power factor. To prevent the power factor from dropping, a high-capacity inductor and high-capacity smoothing capacitor are employed (e.g., Japanese Patent Publication Kokai No. 2001-37281 (FIG. 13)).
However, the first prior art has not proved successful in reducing noise and vibration when applied to a compressor in which a load fluctuation occurs per rotation, because the torque command varies with frequency twice the frequency of the power source and the frequency of the load fluctuation differs from the frequency twice the frequency of the power source. The second prior also has revealed the problem that if the capacity of the inductor or smoothing capacitor is simply reduced with the intention of resource saving or cost reduction, the power factor will drop, giving adverse effects to the power source system.